Spunky has a thoughtful post today on homeschool families' attiitudes towards money and supporting educational businesses. I think these are important points. Many homeschool suppliers are family businesses - meaning that whatever profit they are able to eke out of the sales of their books needs to support an entire family - and sometimes staff as well. It's a very difficult thing to support a family by selling books. I've seen this close-up in helping with my parents' book business (which has downsized considerably over the last year - they used to sell a large quantity of homeschool materials as well).
A lot of people think that the 40% markup retailers get is a lot. But there are a lot of things that bite into this. First of all, a number of books (including a lot of textbooks) have a considerably smaller markup. There are fees to belong to book distribution companies. You have to pay for shipping from the publisher or distributor to the site of your business. If you're selling books at a conference, you also have the expense of renting tables and shipping to that location (and the risk of having to ship some materials back afterwards).
There are homeschoolers out there willing to ignore the virtue of justice when it comes to copyright laws. Breaking these laws hurts the littlest companies (like small Catholic homeschool publishers) the most. At least one small Catholic company has had workbooks returned for reimbursement with photocopied pages accidentally left inside.
It might help to look at it this way. Though the price of books can be quite shocking, buying good books is not only good for your children, but it contributes to the larger cause of having good books available now and in the future. There's a lot of power in "pocketbook voting".